Bis(aminomethyl)cyclohexane is a diamine that has applications as a precursor to an aliphatic diisocyanate (bis(isocyanatomethyl)cyclohexane). It is useful as a chain extender in certain polyurethanes systems and can be used as an epoxy curing agent. Bis(aminomethyl)cyclohexane exists as a number of isomers, of which the 1,3- and 1,4-isomers are of primary interest. The 1,3- and 1,4-isomers can also exist in a number of diastereomeric forms, as the aminomethyl groups can each reside above or below the plane of the cyclohexane ring.
1,3- and 1,4-bis(aminomethyl)cyclohexane mixtures can be prepared via a number of synthetic routes. U.S. Pat. No. 3,143,570 describes a two-step process that requires preparation and isolation of the intermediate solid iminomethylcyclohexanecarbonitriles in water.
As another example, a route may start with butadiene and acrolein, forming 1,2,3,6-tetrahydrobenzaldehyde in a Diels-Alder reaction. This intermediate is then hydroformylated to add a second aldehyde group and reductively aminated to form the desired diamine. A mixture of isomeric forms of the diamine is obtained, as for example, described in the U.S. Pat. No. 6,252,121.
The reductive amination of hydroformylated 1,2,3,6-tetrahydrobenzaldehyde using a sponge-metal catalyst or nickel on silica gel/alumina as in U.S. Pat. No. 6,252,121, however, tends to produce diamine products in low yields. A significant portion of the starting material forms unwanted by-products and polymeric species. As a result, raw material costs may be high and purification of the crude product can be difficult and expensive. Polymeric by-products often foul the reactor and downstream purification unit operations.
It is sometimes possible to suppress by-product formation in reductive amination reactions by “protecting” (or “blocking”) the aldehyde groups with an alkyl amine as, for example, described in the U.S. Pat. Nos. 5,041,675 and 5,055,618. The blocked groups are more resistant to polymerization and other unwanted side reactions. However, this approach requires the use of additional raw materials and introduces additional chemical species into the reaction, which must later be removed from the crude product and recycled. Process yields are still far short of those that are needed to have a highly economical process.
Additionally, the production of 1,3- and 1,4-bis(aminomethyl)cyclohexane via a dialdehyde intermediate may be difficult due to catalyst deactivation that leads to rapidly declining yields. Although more stable catalysts have been identified, these catalysts provide lower yields from the very beginning of operation. In addition, the dialdehyde intermediate route requires a reliable and sufficient supply of acrolein.
In order to overcome these catalyst performance issues and avoid potential future acrolein supply issues, the instant invention provides reductive amination of 1,3- and 1,4-cyanocyclohexane carboxaldehyde (CCA). This intermediate is based on an acrylonitrile feedstock, which is more accessible than acrolein. Simultaneous reduction of the nitrile group and reductive amination of the aldehyde functionality requires a specialized catalyst. Traditional nitrile reduction conditions and catalysts are more aggressive than aldehyde reductive amination conditions and catalysts. Thus, catalysts and conditions that are effective for complete reduction of the nitrile group may also have a tendency to reduce the aldehyde to the corresponding alcohol, resulting in a yield loss. On the other hand, catalysts and conditions that are typically chosen for reductive amination of an aldehyde are typically ineffective in providing complete reduction of the nitrile group, resulting in yield losses to the intermediate aminonitriles. Additionally, the relatively short lifetime of current catalysts introduces other challenges. Catalysts providing good yields to the diamine product (1,3- and 1,4-bis(aminomethyl)cyclohexanes) consistently lose their activity for the nitrile hydrogenation step within less than 250 hours of time on stream. Economically viable catalysts for this process require a much higher number of hours of lifetime, or, equivalently, pounds of 1,3- and 1,4-bis(aminomethyl)cyclohexanes produced per pound of catalyst.
Accordingly, it would be desirable to provide a method by which cycloaliphatic bis(aminomethyl) compounds can be prepared economically and in high yield.